Since macOS High Sierra kVK_RightCommand has been HIToolbox/Events.h. It wasn't defined in the macOS Sierra days. We don't build for macOS Sierra any longer and can remove the workaround.
Actually this constant has been around since macOS Sierra – as the comment in the code stated.
Created attachment 380238 [details]
Comment on attachment 380238 [details]
Clearing flags on attachment: 380238
Committed r250915: <https://trac.webkit.org/changeset/250915>
All reviewed patches have been landed. Closing bug.